Continued execution of accessor commands on a restricted multiple accessor path of an automated data storage library

ABSTRACT

In an automated data storage library having a plurality of accessors which access portable data storage media with respect to storage shelves and data storage drive(s) along a path, a controller, in response to detection of restricted movement of an accessor at a position along the path, determines a range of motion of another accessor along the path which avoids interfering with the accessor having the restricted movement. The controller also determines a limit to commands of a work queue of commands for operating the accessors, the limit extending from and past the position of the accessor having the restricted movement, along the path; and prevents execution of the limited commands. The limited commands may be failed with a “hardware” error.

REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 10/720,886, filedNov. 21, 2003 now U.S. Pat. No. 6,968,257.

FIELD OF THE INVENTION

This invention relates to automated data storage libraries, and, moreparticularly, to libraries having a plurality of accessors whichseparately access and transport portable data storage media with respectto a plurality of storage shelves and at least one data storage drive.

BACKGROUND OF THE INVENTION

Automated data storage libraries provide a means for storing largequantities of data in portable data storage media, such as magnetic tapecartridges, optical disk cartridges, etc., that are stored in a readilyavailable form at storage shelves of the library. The portable datastorage media are accessed and transported with respect to the storageshelves and at least one data storage drive by an accessor, and the datastorage drive(s) transfer data with respect to the portable data storagemedia.

Typically, data stored at portable data storage media of an automateddata storage library, once requested, is needed quickly. Thus, it isdesirable that an automated data storage library continue to be inoperational condition as much as possible, such as the well known“24×7×365” availability. If only a single accessor is supported, anerror to operation of the accessor can cause customer downtime. Hence,the addition of another accessor will aid in reducing customer downtimeand impact. The accessors typically operate on the same path to servethe entire library on the one path, and can work in a “hot standby”configuration, or in a “dual active” or “multiple active” configuration.In a hot standby configuration, only one accessor is active at anysingle time, and another accessor is in standby mode, waiting for anoperational failure to occur on the active accessor. Thus, the standbyaccessor would only take over in the event of a failure of the activeaccessor, and the standby accessor may push the active accessor into aservice bay, and would become the active accessor. Examples ofactive/standby accessors comprise U.S. Pat. No. 5,894,461 and U.S. Pat.No. 6,309,162. In a multiple active configuration, both of the accessorsare active, and move within separate virtual libraries, or sections ofthe automated data storage library, which may overlap. In the event of afailure of one of the accessors, another accessor would push the failedaccessor into a service bay, and would take over for the entireautomated data storage library. An example of multiple active accessorscomprises U.S. Pat. No. 6,304,524.

A key element in reducing customer downtime and impact is to clear theaisle or accessor path in the event of an accessor failure so that thelibrary is fully functional and to allow the other accessor full accessto the whole library. Therefore, if the failing accessor is unable tomove out of the path, another accessor must try to push the failingaccessor into a service bay, so that the another accessor will have fullaccess to the whole library, which eliminates customer downtime.However, there are certain scenarios where failures can halt thelibrary. An example of an error that causes customer downtime is ifthere is a stuck portable data storage media cartridge between theaccessor and a drive or storage shelf. Another example is if a cartridgehas been dropped and is blocking the accessor along the path. The pathis typically a rail, and the cartridge may jam between the accessor andthe rail. In each example, the accessor is failed due to the stuck orjammed cartridge, and it may not be possible to move the failedaccessor, halting the operation of the library.

SUMMARY OF THE INVENTION

Disclosed are an automated data storage library, a controller, a methodand a computer program product for providing continued execution ofaccessor commands on a restricted multiple accessor path of theautomated data storage library. The library comprises a plurality ofstorage shelves for storing portable data storage media; at least onedata storage drive for transferring data with respect to the portabledata storage media; and a plurality of accessors which separately accessand transport portable data storage media with respect to the pluralityof storage shelves and the data storage drive(s), along at least onepath, and which interfere with one another along the path.

In one embodiment, the library controller operates the plurality ofaccessors, and, in response to detection of restricted movement of oneof the accessors at a position along the path, determines a range ofmotion of another of the accessors along the path which avoidsinterfering with the accessor having the restricted movement, at theposition along the path.

In a further embodiment, the automated data storage library additionallycomprises a plurality of frames in sequence along the at least one path,the frames supporting the storage shelves and the data storage drive(s).The library controller additionally determines the frame in which theaccessor having the restricted movement is positioned, to detect theposition along the at least one path of the accessor having therestricted movement.

Still further, the library controller determines the range of motioncomprising limiting motion of the another accessor to the frames of thesequence, extending from a frame of the sequence spaced from the framein which the accessor having the restricted movement is positioned, toone end of the automated data storage library, all in the direction ofthe path toward the another accessor.

In another embodiment, the storage shelves of the automated data storagelibrary are arranged in a plurality of columns along the path; and thelibrary controller additionally determines the column at which theaccessor having the restricted movement is substantially positioned, todetect the position along the at least one path of the accessor havingthe restricted movement.

Still further, the library controller determines the range of motioncomprising limiting motion of the another accessor along the at leastone path, extending from a column spaced from the column at which theaccessor having the restricted movement is positioned, to one end of theautomated data storage library, all in the direction of the path towardthe another accessor.

In a further embodiment, the library controller additionally operatesthe another of the plurality of accessors to attempt to move the failedaccessor, detecting restricted movement by failure to move the failedaccessor, and detecting the position of the another accessor along theat least one path at the failure.

Still further, the failed accessor having the restricted movement,provides a movement failure indication, and the library controllerdetects the restricted movement of one of the plurality of accessors,from a received movement failure indication from the failed accessor.

In another embodiment, the automated data storage library comprises awork queue of commands for operating at least one of the plurality ofaccessors. The library controller detects restricted movement of one ofthe plurality of accessors at a position along the at least one path;determines a limit to commands of the work queue, the limit from andpast the position of the accessor having the restricted movement, alongthe at least one path; and prevents execution of the limited commands.Further, the library controller fails the limited commands with a“hardware” error with respect to host commands to prevent execution ofthe limited commands.

In a still further embodiment, wherein the commands for operatingaccessors comprise at least an origin and a destination, the librarycontroller determines whether either an origin or a destination of acommand of the work queue is beyond the limit, to determine the limit tocommands of the work queue.

In an embodiment wherein the automated data storage library comprises aplurality of frames in sequence along the path, the library controllerdetermines the frame in which the accessor having the restrictedmovement is positioned, and the library controller establishes the limitto commands of the work queue at a frame of the sequence spaced from theframe in which the accessor having the restricted movement ispositioned, the frame spaced in the direction of the at least one pathtoward the another accessor, to determine the limit to commands of thework queue.

In an embodiment wherein the storage shelves are arranged in a pluralityof columns along the path, and the library controller determines thecolumn at which the accessor having the restricted movement issubstantially positioned, the library controller establishes the limitto commands of the work queue at a column spaced from the column atwhich the accessor having the restricted movement is positioned, thecolumn of the limit spaced in the direction of the path toward theanother accessor, to determine the limit to commands of the work queue.

For a fuller understanding of the present invention, reference should bemade to the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of an automated data storagelibrary in accordance with the present invention;

FIG. 2 is a perspective view illustration of the accessors of theautomated data storage library of FIG. 1;

FIG. 3 is schematic representation of a portion of the automated datastorage library of FIGS. 1 and 2; and

FIG. 4 is a flow chart depicting the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention is described in preferred embodiments in the followingdescription with reference to the Figures, in which like numbersrepresent the same or similar elements. While this invention isdescribed in terms of the best mode for achieving this invention'sobjectives, it will be appreciated by those skilled in the art thatvariations may be accomplished in view of these teachings withoutdeviating from the spirit or scope of the invention.

Referring to FIG. 1, an automated data storage library is illustratedhaving an inner “wall” of storage shelves 12 and an outer wall ofstorage shelves 14, which store portable data storage media. The datastorage media typically comprise a self-contained cartridge. Examplesinclude magnetic tape cartridges or optical disk cartridges of varioustypes, such as ROM, WORM, and rewriteable, and in various formats. Foruniversal reference to any of these types of media, the terms “portabledata storage media” are used herein.

The automated data storage library of the present invention is amultiple accessor library, the embodiment of the present inventionillustrated in FIG. 1 having at least two accessors 16 and 17. Theaccessors separately access and transport portable data storage mediawith respect to the plurality of storage shelves 12 and 14 and datastorage drives 18 and 19 for reading and/or writing data on the accessedportable data storage media. In the exemplary automated data storagelibrary, a media import/export port is provided for insertion orretrieval of portable data storage media into or out of the library. Theaccessors 16 and 17 run on rails 22 defining a path in the aisle betweenthe inner wall of storage shelves 12 and the outer wall of storageshelves 14.

An operator interface 28 is provided for allowing a servicing operatoror systems operator to communicate with the automated data storagelibrary.

A library controller 24 and a supplemental or back up controller 25 areprovided to operate the accessors 16 and 17. The library controller 24may comprise a programmable computer processor having computer readablecode embodied therein for controlling the operation of the library, andspecifically the accessors 16 and 17. In an alternative embodiment, anembedded and/or distributed control system may be provided in which thefunctions of the library controllers 24 and/or 25 are distributedamongst a plurality of processors located, for example, at distributedpoints of the library. The library controller typically receivescommands from a host system for access to particular portable datastorage media or to media in particular storage shelves, and to deliverthe data storage media to a data storage drive, and subsequently toreturn the portable data storage media to a storage shelf. The startingor access location of a command is called an origin, and the deliverylocation of a command is called a destination. The library controllertypically has a work queue for queuing commands for accessing andtransporting portable data storage media. Commands for reading and orwriting specific data or data at locations on the selected data storagemedia are typically communicated directly to the data storage drives 18,19.

An example of an automated data storage library 10 is the IBM 3494 DataStorage Library. Another example, with a distributed control system, isthe IBM 3584 Ultra Scalable Tape Library.

The accessors 16 and 17 are illustrated in greater detail in FIG. 2. Theaccessors run horizontally on the rails along the library path by “X”drives 33 (only one drive is shown) and associated bands connected torunning sections 31 of the accessors. A pillar 32 is vertically attachedto each of the running sections 31 of the accessors and rides in a toprail 34 to provide vertical stability. Robotic manipulators 36 aremounted on lifting sections 35, and “Y” motors 30 rotate drive elements40 to move the robotic manipulators 36 along the pillars 32.

In the embodiment illustrated in FIG. 2, each of the roboticmanipulators 36 comprises as components, a set of grippers 37 and 38 anda scanner or reader 39. The manipulator 36 can rotate 180° to switch thegrippers to the opposite sides and can access media at either storageshelves 12 or 14 in FIG. 1. Thus, either gripper can grip a portabledata storage media at an outside storage shelf 14 of FIG. 1 and berotated to deliver the portable data storage media to a data storagedrive 18, 19 at the inside of the library. Alternatively, a separate setof grippers may be provided at opposite sides of the manipulator 36, anddata storage drives provided at opposite sides of the accessor path.

The scanners or readers 39 can read information, such as bar code data,or other labeling information, from the portable data storage media orfrom locations on the library to identify the media or to identify thelocation of the accessor in the library.

Referring additionally to FIG. 3, a controller 24 is illustrated ascoupled to the accessors 16 and 17 by means of communication links 60.Controller 25 may also be coupled to the accessors. The communicationlinks 60 may comprise direct cabling, a bus system, or a wirelessconnection. Alternatively, distributed processors may be mounted at theaccessors and/or data storage drives and/or elsewhere in the library.

Referring to FIGS. 1 and 3, the illustrated embodiment of the library isarranged in a plurality of frames 50, 51, 52, 53, 54 and 55, and thestorage shelves 12 and 14 are arranged in parallel columns 57. Thestorage shelves are disposed in the columns along a “Y” axisperpendicular to the rails 22. One or more of the frames 50–55 alsocontain the data storage drives 18, 19, which are positioned such thatthe accessor grippers 37 and 38 can insert or remove the portable datastorage media at the drives.

Location information 41, such as bar code data, or other labelinginformation, may be provided in the library to be read by a scanner orreader 39 to identify the location of the accessor in the library, forexample, for initialization of the accessor servos. Thus, the accessors16 and 17 will be able to locate desired storage shelves and datastorage drives. Further, the accessors 16 and 17 will be able toidentify their location within the library, for example, by continuoustracking of the accessor servos. Typically, the continuous tracking isboth in the “X” direction and the “Y” direction.

Service bays 44 and 45 are provided at each end of the library. Theaccessors 16 and 17 typically operate on the same path to serve theentire library along the one path, and interfere with one another alongthe path. Alternatively, parallel paths may be provided, but theaccessors still interfere with one another along the paths. Pass-thrualcoves may be provided, possibly with storage shelves, into which anaccessor may be switched, allowing another accessor to pass by along themain path. The accessors 16 and 17 can work in a “hot standby”configuration, or in a “dual active” or “multiple active” configuration.In a hot standby configuration, only one accessor is active at anysingle time, and another accessor is in standby mode, waiting for anoperational failure to occur with the active accessor. The standbyaccessor will be placed in a service bay 44 or 45. Thus, the standbyaccessor would only take over in the event of a failure of the activeaccessor. Referring additionally to FIG. 2, the standby accessor mayemploy a bumper 58, 59 to push the failed active accessor into a servicebay, and would become the active accessor. In a multiple activeconfiguration, both of the accessors 16 and 17 are active, and movewithin separate virtual libraries, or sections of the automated datastorage library, which may overlap. In the event of a failure of one ofthe accessors, another accessor may employ a bumper 58, 59 to push thefailed accessor into a service bay and take over for the whole automateddata storage library.

The present invention is directed to continued execution of accessorcommands on a restricted multiple accessor path. Referring additionallyto FIG. 4, failure of an accessor 16 or 17 of FIGS. 1–3 is sensed instep 70. The failure may be of any type which is not immediatelyrecoverable and which prevents immediate continuance of use of theaccessor. The failure may be sensed by the controller as a failure tosuccessfully complete a command after a required number of retries orother error handling procedures, or may be signaled by the failingaccessor. In step 71, another accessor is operated by the controller 24or 25 to employ bumper 58, 59 to push the failed accessor into a servicebay 44 or 45, as discussed above. Alternatively or additionally, thefailed accessor may first attempt to move into the service bay on itsown.

Step 73 of FIG. 4 detects restricted movement of the failed accessor. Inone embodiment, where the library controller additionally operatesanother of the plurality of accessors to attempt to move the failedaccessor, the detection of the restricted movement is detected byfailure to move the failed accessor. The controller may identify therestricted movement by the expiration of a timer without success inmoving the failed accessor; by a failure for the servo of the anotheraccessor to increment despite the application of power; or by othersignal from the another accessor. In another embodiment, the failedaccessor having the restricted movement, provides a movement failureindication, and the library controller detects the restricted movementfrom a received movement failure indication from the failed accessor.

If, instead, the failed accessor can move or can be moved into a servicebay, step 73 leads to step 75, and another accessor, if standby, becomesthe active accessor, and if one of multiple active accessors, takes overfor the whole automated data storage library, as discussed above.

If step 73 detects that the failed accessor is stuck, such that itsmovement is restricted at a position along the path, the controller, instep 76, detects the position of the accessor along the path. Asdiscussed above, in one example, the servo systems of accessors 16 and17 continually track their current positions. In one embodiment,indicated by step 77, the controller may identify the position of therestricted movement by the indication of the servo system, or by anothersignal, from the another accessor. In one embodiment, the controllerextrapolates the position of the failed accessor from the servoinformation of the another accessor. In another embodiment, indicated bystep 78, the failed accessor having the restricted movement, providesthe servo indication of its servo system to indicate its position. Instill another embodiment, the controller continually tracks the servoposition signals from the accessor before failure, and uses the lastposition at the time of the failure as the position of the failedaccessor.

In one embodiment, illustrated by step 80, the library controller, inresponse to detection of restricted movement of one of the accessors ata position along the path, determines a range of motion of another ofthe accessors along the path which avoids interfering with the accessorhaving the restricted movement, at the position along the path.

In one embodiment, wherein the automated data storage library isarranged as a plurality of frames in sequence along the path, thelibrary controller, in step 80, determines from the servo information,the one of the frames 50–55 of FIGS. 1 and 3 in which the accessorhaving the restricted movement is positioned. The library controllerdetermines the range of motion comprising limiting motion of the anotheraccessor to the frames of the sequence, extending from a frame of thesequence spaced from the one of the frames in which the accessor havingthe restricted movement is positioned, to one end of the automated datastorage library, all in the direction of the at least one path towardthe another accessor. For example, if the accessor having the restrictedmovement is accessor 17, and it is positioned in frame 53, asillustrated in FIG. 3, the range of motion may be determined to beframes 50, 51 and 52.

In another embodiment, wherein the storage shelves of the automated datastorage library are arranged in a plurality of columns along the path;the library controller, in step 80, determines the one of the columns 57of the storage shelves 12 and/or 14 of FIGS. 1 and 3 at which theaccessor having the restricted movement is substantially positioned. Thelibrary controller determines the range of motion comprising limitingmotion of the another accessor along the path, extending from a columnspaced from the column at which the accessor having the restrictedmovement is positioned, to one end of the automated data storagelibrary, all in the direction of the at least one path toward theanother accessor. For example, if the accessor having the restrictedmovement is accessor 17 and it is substantially positioned at column 81,the range of motion may be determined to be from column 82 of frame 52to column 83 of frame 50. The range of motion may be specificallydefined, for example, in terms of the “X” servo positions of the anotheraccessor.

In another embodiment, the library controller 24 of FIGS. 1 and 3detects restricted movement of one of the plurality of accessors at aposition along the path; in step 84 of FIG. 4, determines a limit tocommands of the work queue, the limit from and past the position of theaccessor having the restricted movement, along the path; and, in step85, prevents execution of the limited commands. The limit to commands ofthe work queue may also be based on frames or columns, as above. In step85, commands that are not limited by the restricted accessor areexecuted.

In an embodiment where the library is arranged in a plurality of framesin sequence along the path, the library controller determines the one ofthe frames 50–55 of FIGS. 1 and 3 in which the accessor having therestricted movement is positioned, and the library controller, in step84, establishes the limit to commands of the work queue at a frame ofthe sequence spaced from the one of the frames in which the accessorhaving the restricted movement is positioned, the frame spaced in thedirection of the path toward the another accessor. For example, if theaccessor having the restricted movement is accessor 17, and it ispositioned in frame 53, as illustrated in FIG. 3, the limit to commandsof the work queue may be determined to be frame 53, and any command toframes 53–55 of FIG. 1 for the accessor 16 are prevented in step 85.Commands that are not limited by the restricted accessor and/or arewithin the range of motion are executed. Thus, commands to frames 50–52are executed in step 85.

In an embodiment wherein the storage shelves are arranged in a pluralityof columns along the path, the library controller determines the one ofthe columns at which the accessor having the restricted movement issubstantially positioned, and the library controller, in step 84,establishes the limit to commands of the work queue at a column spacedfrom the column at which the accessor having the restricted movement ispositioned, the column of the limit spaced in the direction of the pathtoward the another accessor. For example, if the accessor having therestricted movement is accessor 17, and it is positioned substantiallyat column 81, as illustrated in FIG. 3, the limit to commands of thework queue may be determined to be from column 86, and any command foraccess to column 86 or toward service bay 45 of FIG. 1 by the accessor16 are prevented in step 85. Commands that are not limited by therestricted accessor are executed. Thus, commands to column 82 or towardservice bay 44 are executed.

In an example, where the commands for operating the accessors compriseat least an origin and a destination, the library controller determineswhether either an origin or a destination of a command of the work queueis beyond the limit of the commands of the work queue.

In one example, the library controller fails the limited commands with a“hardware” error to prevent execution of the limited commands. Commandstypically originate with a host system (via a SCSI, Fibre Channel, etc.,communication system), an operator panel or service panel, or over theweb. A “hardware” error typically is a specific response within theprotocol of the communication system. For example, in SCSI protocol(also for Fibre Channel) a “hardware” error is signaled with a “04”sense key and a “4400” additional sense codes (ASC) and additional sensecode qualifiers (ASCQ).

The method of the present invention may be carried out by a computerprogram product usable with the programmable computer processor havingcomputer readable code embodied therein, comprising controller 24 and/orcontroller 25 of FIGS. 1 and 3. The computer program product may bestored on a storage medium, such as a CD-ROM, or magnetic tape, etc., ormay be supplied from a host system to the controller(s).

The illustrated components of the automated data storage library ofFIGS. 1–3 may be varied, combined, or combined functions may beseparated, as is known to those of skill in the art. The illustratedsteps of FIG. 4 may be altered in sequence, omitted, or other stepsadded, as is known to those of skill in the art.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations to those embodiments may occur to one skilled in the arewithout departing from the scope of the present invention as set forthin the following claims.

1. An automated data storage library, comprising: a plurality of storageshelves for storing portable data storage media; at least one datastorage drive for transferring data with respect to said portable datastorage media; a plurality of accessors which separately access andtransport portable data storage media with respect to said plurality ofstorage shelves and said at least one data storage drive, along at leastone path, and which interfere with one another along said at least onepath; a work queue of commands for operating at least one of saidplurality of accessors; and a library controller for operating saidplurality of accessors, said library controller: detects restrictedmovement of one of said plurality of accessors at a position along saidat least one path; determines a limit extending to commands of said workqueue, said limit from and past said position of said accessor havingsaid restricted movement, along said at least one path; and preventsexecution of said limited commands.
 2. The automated data storagelibrary of claim 1, wherein said library controller fails said limitedcommands with a “hardware” error to prevent execution of said limitedcommands.
 3. The automated data storage library of claim 1, wherein saidcommands for operating said at least one of said plurality of accessorscomprise at least an origin and a destination, and wherein said librarycontroller determines whether either an origin or a destination of acommand of said work queue is beyond said limit, to determine said limitto commands of said work queue.
 4. The automated data storage library ofclaim 3, wherein said automated data storage library comprises aplurality of frames in sequence along said at least one path; andwherein said library controller additionally determines the one of saidframes in which said accessor having said restricted movement ispositioned, to detect said position along said at least one path of saidaccessor having said restricted movement.
 5. The automated data storagelibrary of claim 4, wherein said library controller establishes saidlimit to commands of said work queue at a frame of said sequence spacedfrom said one of said frames in which said accessor having saidrestricted movement is positioned, said frame spaced in the direction ofsaid at least one path toward said another accessor, to determine saidlimit to commands of said work queue.
 6. The automated data storagelibrary of claim 3, wherein said storage shelves are arranged in aplurality of columns along said at least one path; and wherein saidlibrary controller additionally determines the one of said columns atwhich said accessor having said restricted movement is substantiallypositioned, to detect said position along said at least one path of saidaccessor having said restricted movement.
 7. The automated data storagelibrary of claim 6, wherein said library controller establishes saidlimit to commands of said work queue at a column spaced from said columnat which said accessor having said restricted movement is positioned,said column of said limit spaced in the direction of said at least onepath toward said another accessor, to determine said limit to commandsof said work queue.
 8. A controller for operating a plurality ofaccessors of an automated data storage library which separately accesssaid automated data storage library along at least one path, and whichinterfere with one another along said at least one path, said automateddata storage library having a work queue of commands for operating atleast one of said plurality of accessors, said controller: detectsrestricted movement of one of said plurality of accessors at a positionalong said at least one path; determines a limit extending to commandsof said work queue, said limit from and past said position of saidaccessor having said restricted movement, along said at least one path;and prevents execution of said limited commands.
 9. The controller ofclaim 8, wherein said controller fails said limited commands with a“hardware” error to prevent execution of said limited commands.
 10. Thecontroller of claim 8, wherein said commands for operating said at leastone of said plurality of accessors comprise at least an origin and adestination, and said controller determines whether either an origin ora destination of a command of said work queue is beyond said limit, todetermine said limit to commands of said work queue.
 11. The controllerof claim 10, wherein said automated data storage library comprises aplurality of frames in sequence along said at least one path; andwherein said controller additionally determines the one of said framesin which said accessor having said restricted movement is positioned, todetect said position along said at least one path of said accessorhaving said restricted movement.
 12. The controller of claim 11, whereinsaid controller establishes said limit to commands of said work queue ata frame of said sequence spaced from said one of said frames in whichsaid accessor having said restricted movement is positioned, said framespaced in the direction of said at least one path toward said anotheraccessor, to determine said limit to commands of said work queue. 13.The controller of claim 10, wherein said automated data storage librarycomprises a plurality of storage shelves for storing portable datastorage media for access by said plurality of accessors, said storageshelves arranged in a plurality of columns along said at least one path;and wherein said controller additionally determines the one of saidcolumns at which said accessor having said restricted movement issubstantially positioned, to detect said position along said at leastone path of said accessor having said restricted movement.
 14. Thecontroller of claim 13, wherein said controller establishes said limitto commands of said work queue at a column spaced from said column atwhich said accessor having said restricted movement is positioned, saidcolumn of said limit spaced in the direction of said at least one pathtoward said another accessor, to determine said limit to commands ofsaid work queue.
 15. A method for operating a plurality of accessors ofan automated data storage library which separately access said automateddata storage library along at least one path, and which interfere withone another along said at least one path, said automated data storagelibrary having a work queue of commands for operating at least one ofsaid plurality of accessors, said method comprising the steps of:detecting restricted movement of one of said plurality of accessors at aposition along said at least one path; determining a limit extending tocommands of said work queue, said limit from and past said position ofsaid accessor having said restricted movement, along said at least onepath; and preventing execution of said limited commands.
 16. The methodof claim 15, wherein said step of preventing execution of said limitedcommands additionally comprises failing said limited commands with a“hardware” error.
 17. The method of claim 15, wherein said commands foroperating said at least one of said plurality of accessors comprise atleast an origin and a destination, and said step of determining a limitto commands of said work queue comprises determining whether either anorigin or a destination of a command of said work queue is beyond saidlimit.
 18. The method of claim 17, wherein said automated data storagelibrary comprises a plurality of frames in sequence along said at leastone path; and wherein said step of detecting said restricted movement ofone of said plurality of accessors at a position along said at least onepath, additionally comprises determining the one of said frames in whichsaid accessor having said restricted movement is positioned.
 19. Themethod of claim 18, wherein said step of determining a limit to commandsof said work queue comprises establishing said limit at a frame of saidsequence spaced from said one of said frames in which said accessorhaving said restricted movement is positioned, said frame spaced in thedirection of said at least one path toward said another accessor. 20.The method of claim 17, wherein said automated data storage librarycomprises a plurality of storage shelves for storing portable datastorage media for access by said plurality of accessors, said storageshelves arranged in a plurality of columns along said at least one path;and wherein said step of detecting said restricted movement of one ofsaid plurality of accessors at a position along said at least one path,additionally comprises determining the one of said columns at which saidaccessor having said restricted movement is substantially positioned.21. The method of claim 20, wherein said step of determining a limit tocommands of said work queue comprises establishing said limit at acolumn spaced from said column at which said accessor having saidrestricted movement is positioned, said column of said limit spaced inthe direction of said at least one path toward said another accessor.22. A computer program product usable with at least one programmablecomputer processor having computer readable code embodied therein, saidat least one programmable computer processor for operating a pluralityof accessors of an automated data storage library which separatelyaccess said automated data storage library along at least one path, andwhich interfere with one another along said at least one path, saidautomated data storage library having a work queue of commands foroperating at least one of said plurality of accessors, said computerprogram product comprising: computer readable program code causing saidat least one programmable computer processor to detect restrictedmovement of one of said plurality of accessors at a position along saidat least one path; computer readable program code causing said at leastone programmable computer processor to determine a limit extending tocommands of said work queue, said limit from and past said position ofsaid accessor having said restricted movement, along said at least onepath; and computer readable program code causing said at least oneprogrammable computer processor to prevent execution of said limitedcommands.
 23. The computer program product of claim 22, wherein saidcomputer readable program code causing said at least one programmablecomputer processor to prevent execution of said limited commands,additionally comprises computer readable program code causing said atleast one programmable computer processor to fail said limited commandswith a “hardware” error.
 24. The computer program product of claim 22,wherein said commands for operating said at least one of said pluralityof accessors comprise at least an origin and a destination, and whereinsaid computer readable program code causing said at least oneprogrammable computer processor to determine a limit to commands of saidwork queue, comprises computer readable program code causing said atleast one programmable computer processor to determine whether either anorigin or a destination of a command of said work queue is beyond saidlimit.
 25. The computer program product of claim 24, wherein saidautomated data storage library comprises a plurality of frames insequence along said at least one path; and additionally comprisingcomputer readable program code causing said at least one programmablecomputer processor to determine the one of said frames in which saidaccessor having said restricted movement is positioned, to detect saidposition along said at least one path of said accessor having saidrestricted movement.
 26. The computer program product of claim 25,wherein said computer readable program code causing said at least oneprogrammable computer processor to determine a limit to commands of saidwork queue comprises establishing said limit at a frame of said sequencespaced from said one of said frames in which said accessor having saidrestricted movement is positioned, said frame spaced in the direction ofsaid at least one path toward said another accessor.
 27. The computerprogram product of claim 24, wherein said automated data storage librarycomprises a plurality of storage shelves for storing portable datastorage media for access by said plurality of accessors, said storageshelves arranged in a plurality of columns along said at least one path;and wherein said computer readable program code causing said at leastone programmable computer processor to detect said restricted movementof one of said plurality of accessors at a position along said at leastone path, additionally comprises determining the one of said columns atwhich said accessor having said restricted movement is substantiallypositioned, to detect said position along said at least one path of saidaccessor having said restricted movement.
 28. The computer programproduct of claim 27, wherein said computer readable program code causingsaid at least one programmable computer processor to determine a limitto commands of said work queue comprises establishing said limit at acolumn spaced from said column at which said accessor having saidrestricted movement is positioned, said column of said limit spaced inthe direction of said at least one path toward said another accessor.